A typical virtualization system consists of a guest operating system (OS) running within a virtual machine (VM). The VM is emulated by a hypervisor as a physical machine to the guest OS. The VM is usually controlled by a guest administrator. In addition, a virtualization system includes a hypervisor that manages one or more VMs on a single physical machine (a host). The hypervisor controls how each VM will behave. Lastly, a virtualization system may include a virtualization management tool that controls one or more hypervisors. This virtualization management tool may be used by a datacenter manager.
As with physical machines, VMs can differ significantly in the kind of hardware available to them, which can be emulated hardware or real host hardware that the hypervisor makes accessible to the VM. Examples of hardware that is emulated include CPU features, message signaled support in a network card, amount of RAM, etc. In some cases, this emulated hardware cannot change for a VM if the guest OS of the VM is running.
Sometimes, management may give a command that requires VM hardware different from what is currently presented to the VM. For example, during live migration, a VM is moved between hypervisors. In some cases, there may be hypervisors or physical machines that might not support a feature that is currently in use by the VM hardware or may not support the hardware itself.
If VM hardware is not supported, this typically means that changes and/or updates are needed to the VM hardware. Currently, such changes require VM downtime for shutting down the VM and restarting it with new VM hardware. These changes also require that a datacenter manager should have access to the VM with a VM administrator role, which may result in security issues. Another current solution is to have redundant VMs at the VM level or present redundant VM hardware for a VM, so that a VM can be updated without resulting in a noticeable shutdown of the VM to an end user. However, this kind of redundancy is often impossible or it complicates life for a VM administrator. Yet another solution is to implement a hot plug capability where a VM is notified of hardware changes and then the VM must acknowledge this before moving on. However, this technique may only work for some VM hardware/VM OS configurations. It also requires VM cooperation which can be time-consuming, and involves some downtime while the device is reconfigured.